Magnesium alloys



United States Pate 3,071,462 Fatented Jan. 1, 1963 The present application is a continuation-in-part of application Serial No. 768,062, filed on October 20, 1958, by me and assigned to the assignee of the present application.

This invention relates to magnesium alloys suitable, inter alia, for use as a canning material for the fuel elements of nuclear reactors. For such purpose, the alloy initially cast is required to be subjected to mechanical working whereby it is wrought into a form suitable for the required application. This may be effected by hot extrusion.

The important mechanical properties required in an alloy to be used as a canning material for uranium fuel in a thermal nuclear reactor are high creep ductility between 200 C. and 500 C.; small grain size and high stability of grain size up to 500 C.; freedom from excessive intercrystalline cavitation; high creep strength at low stresses at the highest operating temperatures and good weldability.

Alloys used for cans in natural uranium reactors must also possess very low neutron absorption cross-sections and for this reason magnesium is the most important metal to form the main constituent of any alloy used. Any alloying elements used must be such that the neutron absorption cross-section of the resulting alloy is adequately small. Magnesium alloys which have been considered up to now have been those in which the main alloying constituent is approximately 1% aluminum, magnesium alloys containing 0.5% zirconium, and those containing 0.5% zirconium and 0.51% zinc.

The two alloys of magnesium with 0.5% zirconium, and magnesium with 0.5 zirconium and 0.5%-1% zinc have many properties which make them suitable for reactor cans with the exception that their creep strength at high temperatures of the order 500 C. is insufiiciently high.

I have found that a small quantity of manganese alloyed in magnesium with zirconium produces an alloy with high creep resistance, yet still possessing the other desirable properties which are known to exist in binary magnesium-zirconium or ternary magnesium-zirconiumzinc alloys. These improved alloys are basically singlephase alloys, which at high temperatures do not exhibit precipitation hardening which is usually associated with creep resistance.

An alloy according to the invention consists of zirconium, magnesium and manganese, the proportions, by weight, in the alloy being 0.5l.0% Zr, 0.050.5% Mn, and the remainder magnesium.

It is known that the useful liquid and solid solubility of Zr in Mg to form a single phase alloy is about 1.0%, above which percentage a zirconium precipitate is present in the final alloy. Also manganese has a limited liquid and solid solubility in magnesium, such that if the amount present in a binary alloy exceeds about 0.5% an undesirable precipitate is present in the alloy, which may lead to embrittlement. If Zr and Mn are both present in an Mg alloy, each of the two elements limits the solubility of the other, Le. a small Mn addition of a few tenths of 1% reduces the solubility of Zr to less than 1.0%. The incorporation of Mn in the manufacture of Mg-Zr alloys therefore necessitates the reduction of the Zr content to less than 1.0%, both on grounds of obtaining the alloy elements in solution in the melt, and in producing a final alloy of single phase, which for the latter reason should also be free from serious embrittlement. The Zr and Mn contents of any Mg-Zr-Mn alloys are, therefore, interdependent.

Furthermore, the preferred maximum Mn content for a canning material on grounds of neutron absorption economy is about 0.2%.

In more detail, the following examples are given of preferred compositions which were found to be basically single phase alloys.

Alloy 1): 0.6% Zr, 0.1% Mn, remainder Mg. Alloy (2): 0.7% Zr, 0.27% Mn, remainder Mg.

The improvement in creep resistance of alloy (1) containing manganese over its parent alloy of MgZr and over alloy (A) of lower content of zirconium is shown by tensile test creep results as follows:'

Composition Secondarycreep rate at 1,000 hours duration at 450 C. at

stress of Alloy Z Parent Mg.Zr Alloy A Alloy 1 Composition,

Secondary creep percent;

rate at 1,000 hours duration at 500 C., strain/hour Stress, p.s.i.

Average Percent Percent grain tensile tensile Initial diameter in elongation elon ation Alloy average microns at fracture at fracture grain diamafter when when eter in annealing strained at strained at microns 500 hours 1% per hr. 0.1% per hr.

at 500 C. at 200 C. at 200 C.

Parent Mg.Zr 32 97 77 76 Alloy (1) 25 35 49 41 Alloy (2) 37 60 41 What I claim is: 1. An alloy consisting of magnesium, zirconium and 3 manganese, the proportions, by weight, of the constituents of the alloy being 0.6 to 0.7% zirconium, 0.1 to 0.27% manganese, and the remainder wholly magnesium except for unavoidable impurities.

2. An alloy consisting essentially, by weight, of zirconium 0.6%, manganese 0.1%, and the remainder magnesium except for unavoidable impurities.

3. An alloy consisting essentially, by weight, of zirconium 0.7%, manganese 0.27%, and the remainder magnesium except for unavoidable impurities.

References Cited in the file of this patent FOREIGN PATENTS Great Britain Dec. 17, 1958 OTHER REFERENCES Influence of Zirconium upon the solidification of Magnesium Alloys and Some of the Properties of Magnesium Alloys Containing Zirconium, by Franz Sauerwald, Zeitschrift fiir Metallkunde, vol. 40, 1949, pp. 44 and 45 

1. AN ALLOY CONSISTING OF MAGNESIUM, ZIRCONIUM AND MANGANESE, THE PROPORTIONS, BY WEIGHT, OF THE CONSTITUENTS OF THE ALLOY BEING 0.6 TO 0.7% ZIRCONIUM, 0.1 TO 0.27% MANGANESE, AND THE REMAINDER WHOLLY MAGNESIUM EXCEPT FOR UNAVOIDABLE IMPURITIES. 